Chirone, Roberto; (2018) A study of the effect of process conditions on the fluidization behaviour of cohesive industrial powders linked with rheological studies. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The role that fluidized bed reactors and other unit operations play for a wide range of industries is well recognized. Although fluidized bed systems offer several advantages such as high heat transfer rate, rapid solids mixing, large surface contact, high heat and mass transfer rates between gas and particles, a complete understanding of the phenomena occurring in these reactors is still a challenge, with reference to the role of the process conditions, such as pressure, temperature and humidity. Generally, the temperature affects both the properties of the material and the fluid, such as density and fluidizing gas viscosity. These changes can influence significantly the design and efficiency of the reactor. For these reasons, the effect of the temperature on fluidization became the center of a significant academic effort aimed at providing a theoretical framework to underpin the major physical phenomena involved and, in particular, to develop correlations for the scale-up of fluidized bed reactors. Several works have demonstrated that process conditions can influence the role of the interparticle forces (IPFs) in the fluidization behaviour of powders. Given the complexity of the phenomena involved, a direct quantification of the particle-particle interactions in fluidized beds and of their changes at process conditions is very difficult. Within this framework, powder rheology represents an appealing tool to evaluate indirectly the effects of the interparticles forces on fluidization. The main objective of the present work is to provide a basis for understanding the factor responsible for changes in fluidization behaviour of industrial particles under realistic process conditions. In order to address the problem of assessing the fluidization behavior of powders at high temperature, a multidisciplinary approach linking micro and macro properties of the particulate system is adopted in this project. On the one hand, the investigation of the fluidization behavior at process conditions is carried out by means of standard fluidization tests; on the other hand, the characterization of the flow properties of the same powders is performed by means of powder rheology tests. To this end, the 5 experimental campaign was performed using a 140x1000 mm heated gas fluidized bed and a modified Schulze annular shear cell. Both experimental apparatuses allowed a safe operation of the system up to 600 °C. Five cuts of the same mother particles covering Group B, A and C of Geldart’s classification were investigated over a range of temperatures from ambient to 500 °C. Furthermore, two reacted samples of the same mother particles but, containing different levels of impurities were tested. Shear test experiments show changes of the flow properties at high temperatures. The powder cohesion is the parameter which appears to be mostly affected by temperature while the angle of internal friction shows a weaker dependence on temperature and consolidation level. A model combining the continuum approach and the particle–particle interaction description was used to correlate the powder tensile strength with the interparticle forces. In the presence of only van der Waals forces, the model with the assumption of plastic deformation at contact points and a reasonable value of the mean curvature radius is able to predict the correct order of magnitude of the tensile strength. Furthermore, the significant increase of the cohesion of the reacted material with increasing temperature can be only justified by considering an active role of capillary bridges between the particle asperities. These findings, together with the nature of the impurities characterized by means of EDX analysis applied to SEM imaging, strongly suggest that the observed changes for the reacted material are due to the occurrence of capillary bridges between particles, even if thermal analyses are not able to detect any significant phase changes. This work assessed also the validity of some classical concepts and equations commonly used for describing the fluidization behaviour at low and high temperature. The minimum fluidization conditions were well predicted by the Ergun equation when accounting for the experimental values of the bed voidage. The bed collapse test was used to quantify changes in the aeratability of the powders between low and high temperature and to identify the minimum bubbling conditions. For systems dominated by IPFs the analysis of the voidage of the dense phase and the overall bed expansion as a function of the flow rate allowed reconstructing the sequence of phenomena through which a stable flow of bubbles across the solid mass were achieved. 6 The role of hydrodynamic forces and of interparticle forces on the fluidization behaviour of the particulate systems studied was investigated by looking at the applicability of the Foscolo and Gibilaro stability criterion [Chem Eng Sci. 1984, 39 (12): 1667-1675]. In particular the analysis followed the approach indicated by Valverde et al. [Europhys Lett. 2007, 54: 329-334.], which makes use of the initial settling velocity of cohesive particles and of the Bond number derived from rheometry results. The results of the analysis show the capability of predicting the final structure of the bed with temperature when considering an aggregative fluidization behaviour caused by interparticle adhesive forces. These results also suggest the potential use of the powder rheometry carried out at high temperature as a sensitive method to detect phase changes in particulate systems that are limited to the particle surface that can significantly affect the working conditions of fluidized bed reactors. More in general, the results indicate that shear testing results at ambient and high temperatures allow to correctly estimate the intensity of interparticle forces in particulate systems.

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